Several types of upright, ultra low temperature freezers are available, for example, for cooling various scientific products to very low temperatures. One such upright freezer has been available since prior to this invention from Forma Scientific, Inc., located in Marietta, Ohio, under their "8500" and "900" series of freezers. These upright freezers use various controls but each traditionally included a single inner chamber cooled by a surrounding "cold wall" type refrigeration system operated by an ultra low temperature refrigeration unit disposed in the base of the freezer. Evaporation or cooling coils run up and down the side walls and back wall of the freezer as well as through the top and bottom walls. No fans are used for cooling the freezer chamber. Cooling is only accomplished by conduction and convection from the freezer walls into the chamber. A single full length upright door is mounted by hinges in a conventional fashion to the front of the freezer to provide access to the chamber. This particular freezer is capable of cooling the chamber to temperatures within the range of about -50.degree. C. to -86.degree. C.
A problem which has arisen with such ultra low temperature freezers is that when the front door is opened, the extremely cold and heavy air within the chamber tends to spill out of the bottom of the chamber through the front opening of the freezer. Often, these freezers are used for both "long term" and "short term" items which must both be maintained at the same low temperature. Often, short term items must be accessed in a repeated fashion throughout the day and therefore the extremely cold and relatively heavy air, when compared to ambient air, sinks to the bottom of the freezer and spills or falls out the bottom of the front door opening resulting in a substantial loss of cold air every time the door is opened. This not only undesirably increases the temperature of the freezer chamber and its contents, but places increased loads on the refrigeration unit as it must operate on a more continuous basis to account for all of the lost cold air from within the chamber.
It would therefore be desirable to provide an ultra low temperature freezer which allows ready access to both long term and short term items maintained therein at the same temperature but which prevents spillage of cold air from the bottom of the chamber when short term items are repeatedly accessed.